The present invention is directed to the removal of species from aqueous solutions. In particular, the present invention is directed to the removal of species including an arsenic [III] component from groundwater in purification for drinking.
Ion exchange beds are used to remove toxic ions from solutions. In general, ion exchange beds use columns of polymeric material with suitable ion exchange sites, such as sulphonic acid groups or quaternary ammonium salts, which are grafted onto a polymer matrix. The polymeric material is often in the form of beads, which are usually about the size of a grain of rice. The ion exchange process, such as the exchange of chloride ions on the surface of the beads for nitrate ions in an aqueous solution, occurs mainly at the surface of the bead. Consequently, the capacity of an ion exchange column is a function of the number of beads and the available ion exchange sites on the surface of individual beads. Very little of the ion exchange capacity on the inside of the bead is utilized. As a consequence, the capacity of an ion exchange column is typically much smaller than the total number of chemical reactions the ion exchange media could theoretically undergo.
The rate of the ion exchange reaction is further slowed as the reaction depends on the diffusion of the toxic ions into and out of the resin. The concentration of the toxic ions will be lower at the surface of the resin than in the bulk of the liquid. Each section of the column may be considered as an area where an equilibrium is set up between the leaving ion and the entering ion at their relative concentrations. As an aqueous solution moves up or down the ion exchange column and is exposed to clean resin surfaces, the ion exchange reaction is more rapid. This process is analogous to distillation and the concept of theoretical plates. Consequently, when ion exchange columns are used to remove toxic ions from liquid solutions, those skilled in the art favor longer columns to increase the time the aqueous solution is in contact with the ion exchange media.
In order to allow the aqueous solution containing the toxic ions to flow through the column and come into contact with as many ion exchange sites on the resin as possible, those skilled in the art further prefer the ion exchange media to have good hydraulic permeability throughout. Consequently, when zirconia has been used as a component in ion exchange media, those skilled in the art have modified the zirconia, ice. by creating bead like particles using polymeric compounds, to increase the hydraulic permeability of the column. While these modifications to the zirconia increase the ion exchange media""s hydraulic permeability, they limit the media""s capacity as fewer ion exchange sites are accessible to the toxic ions in the aqueous solution.
Arsenic is a toxin that may be found in various types of water from commercial effluents to naturally occurring groundwater. The presence of arsenic in water creates difficulties in the removal processes as well as in disposal of the media or regeneration solution. Recently, the presence of arsenic in groundwater has lead to a crisis in the Bengal Basin. As described in an article from CandEN, pp. 128-132, Dec. 6, 1999, the United Nations International Children""s Emergency Fund (UNICEF) installed millions of wells in Bangladesh villages in the 1970""s in an attempt to provide the frequently flood ravaged country with safe drinking water. The program was an early success, drastically reducing instances of cholera and other diseases in the country. Unfortunately, the groundwater was contaminated and people began to suffer from arsenic poisoning.
The World Health Organization sets the standard of acceptable arsenic levels in drinking water at 50 parts per billion (ppb). New laws may set this standard even lower, to levels not greater than 10 ppb. In view of poor testing methods and without the means to achieve even the current standards of safe arsenic levels in Bangladesh, the tragedy continues in that region.
Arsenic exists in two soluble and dangerous oxidation states, As+3, which is known as arsenite and As+5, which is known as arsenate. Both forms are toxic and exist in groundwater, although arsenite is the more lethal and the more difficult to remove.
The Environmental Protection Agency (EPA) and others believe that arsenite predominates in aquifers due to a lack of oxygen and oxidizing species that can convert the arsenite to arsenate. This is not the case with surface waters where oxygen is plentiful and there are other ions such as ferric ions which can complete the oxidation. It has been believed that arsenite cannot be as easily removed as the arsenate ion because arsenite is only very slightly ionized in water. The ionization constant for arsenite is only 5xc3x9710xe2x88x9210 compared to the first ionization constant of arsenate, 5.6xc3x9710xe2x88x923. Some processes chemically oxidize the arsenite to arsenate to facilitate its removal, but this creates an additional step in the removal process.
There have been various proposals for removing arsenic from drinking water including precipitation with iron or copper and attempts to immobilize the arsenic with biological agents. These approaches have various problems, not the least of which is the difficulty of removing arsenic to the very low levels that are required for safe drinking, on the order of parts per billion. Other difficulties are that other competing ions for ion exchange sites or ions unnecessarily precipitated, are on the order of parts per million, a thousand times more material than arsenic species. Large amounts of non-toxic and beneficial ions are removed along with the very much smaller amounts of arsenic. Accordingly, the removed material is bulkier than necessary, toxic and a disposal problem.
There is a need for an efficient, economical, and high capacity water treatment method and apparatus for removing toxic ions from aqueous solutions and from groundwater in particular. This process should be capable of removing arsenic to levels not greater than 10 ppb. The media should be capable of being regenerated repeatedly with little loss in capacity, to reduce costs and provide for continuous use of wells. The system must be safe and prevent loss of removed arsenic during operation. There must be a minimum amount of waste and the device should be suitable for use in low pressure applications such as wells. Finally, the system should be easy to use and inexpensive to obtain and operate. A step change in technology is required to satisfy all of these needs.
The present invention advantageously satisfies the above needs. Arsenite and arsenate may be removed to safe levels not greater than 10 ppb and even to 1 ppb levels, using a ceramic media of the present invention. The inventive media is nontoxic, insoluble and chemically and biologically stable. The media has a very high affinity not only for ionic arsenate species, but also for the soluble and nonionic arsenite species. In view of its stability, the inventive media may be used over and over again. Little or no loss in capacity has been observed and the process of stripping arsenic from the ceramic creates a minimum secondary waste.
Arsenite removal occurs in an anomalous way in the inventive media. The notoriously difficult-to-remove arsenite is much more easily removed at a high capacity compared to arsenate. This is surprising in that arsenite is poorly ionized. When the media reaches full capacity the outlet concentration may be equal to the input concentration with no spiking that is observed with ion exchange systems. The inventive media is not strictly an ion exchange material in the normal sense of the term, though it may be used to remove certain ionic species. The inventive media behaves differently than an ion exchange media since it also has the ability to remove nonionic species very well. Removal of arsenite and arsenate is achieved using benign chemical solutions.
The inventive media will remove nonionic arsenite in addition to certain ionized species such as chromate. Advantageously, however, the media is selective in that it shows little or no interference from species including iron, sulfate, nitrate, chloride and bicarbonate. Such species may be present in much higher concentrations than arsenic. Many sites would be lost and a greater amount of residue would be created during regeneration if the media were to react with these ions over arsenite.
In the present invention the media includes a material selected from the group consisting of zirconium hydroxide, titanium hydroxide, hafnium hydroxide, and combinations thereof. The media may be efficiently and economically used in a water treatment apparatus for domestic and commercial systems, including point of use drinking water treatment. The term xe2x80x9cmediaxe2x80x9d as used herein is meant to refer to the active composition that removes the intended species, such as the As+3 species, from water. This term is not intended to be limited to a certain form or shape of media. The media may take the form of a powder, although other forms of the media may also be suitable. Preferably, the media is a powder having a water content characterized by a loss on ignition of at least 40% when heated at 1000xc2x0 C. for one hour. The media is preferably used as a layer with an aspect ratio greater than that of a column. The aspect ratio of the layer is at least 1:1, more preferably, at least about 10:1. The preferred aspect ratio of at least about 10:1 results in devices having efficient flow of solution therethrough suitable for residential and other commercial applications. The term xe2x80x9caspect ratioxe2x80x9d is used herein to refer to the area of the layer that is exposed to the liquid to be treated, divided by the thickness of the layer through which the liquid must travel in the process of removing intended species therefrom. The zirconium hydroxide powder may have a density upon tamping to constant volume, of at least 0.7 g/ml. The media need not be applied to other supports such as resin beads, charcoal, and the like. Such forms of media typically require heating the media, which undesirably diminishes the activity of its active component.
A preferred use of the present invention is in the treatment of water, such as groundwater, so as to make it suitable for drinking. Groundwater has different competing ions that render its treatment unique, especially in the removal of arsenic compounds in view of their relatively small concentration. The present invention may be suitable for use in treating other aqueous solutions as well, such as commercial effluents. The invention may be used to remove a variety of ions from aqueous solutions such as arsenate, selenate, chromate, borate, perchlorate, fluoride and the like. The inventive zirconium hydroxide media has a greater affinity for some of these species than others. With respect to arsenic, the arsenite form is much more strongly adsorbed than arsenate. This is quite unexpected since the form of arsenite removed from groundwater is nonionic. Therefore, the ion exchange function would not be expected to have an effect on the removal of arsenite. It has further been discovered that arsenic compounds are more strongly adsorbed than most other oxyanions except chromate. This surprising discovery increases the efficiency and utility of the present invention because sulfate, chloride and bicarbonate ions usually exist in well water in much higher concentrations than arsenic.
These and other objects of the invention can be achieved in part because, surprisingly, it has been found that zirconium hydroxide, particularly in its freshly prepared form (from aqueous medium), has a much greater affinity for toxic ions and species than had previously been assumed. While not wanting to be bound by theory, it is believed that this affinity is the result of a ligand effect between the zirconium hydroxide, especially when freshly prepared and significantly hydrated, and the species in a contaminated aqueous solution. Because of this unexpected increased affinity, a long ion exchange bed is unnecessary and even undesirable.
A preferred aspect of the present invention is directed to the use of a layer of media having an aspect ratio of at least 1:1, more preferably, at least about 10:1, to remove species comprising a nonionic arsenite (As+3) component from water. The media comprises a material selected from the group consisting of zirconium hydroxide, titanium hydroxide, hafnium hydroxide and combinations thereof. Preferred media materials include at least one of zirconium hydroxide and titanium hydroxide. The water is exposed to the media effective to remove the As+3 component to low levels, preferably to a level not greater than 10 ppb and even to a level not greater than 1 ppb. The inventive media is capable of removing other species as well, including ions selected from the group consisting of arsenate, selenate, chromate, borate, perchlorate, fluoride and mixtures thereof. However, a preferred aspect of the invention, is the removal of the nonionic As+3 component, which is notoriously difficult to remove in view of it uncharged character.
The present invention overcomes the typical difficulties in removing As+3 components. Some processes that seek to remove As+3 species first convert the species to As+5, which is easier to remove. This indirect removal of As+3 species is disadvantageous in that additional steps are required. Other competitive technologies such as activated alumina require the use of extended columns which may be considered bulky in certain applications. The inventive media is advantageous in that it may be regenerated repeatedly with little or no loss in capacity. This will minimize costs and provide for continuous use of wells being purified. The present invention operates safely, without significant loss of captured arsenic over intended periods of use. There is a minimal amount of waste. Moreover, the present invention may be used in a low pressure device. This is unexpected in that, in the case of the powder form of the media, one would believe that high pressures are required to achieve suitable flow rates through a powder. The present invention achieves degrees of As+3 removal, efficiency and safety that heretofore have not been obtainable. The present media shows little or no interference from some ionic species that occur in much greater concentrations than arsenite. As a result, the present invention does not suffer from excessive loss of reactive sites that would occur upon reaction with such ionic species, nor does it suffer from difficulties in regenerating such excessive residues.
Referring now to more specific features of the present invention, one embodiment of the invention is directed to a method of treating an aqueous solution, comprising passing an aqueous solution into contact with a layer of media having an aspect ratio of at least 1:1, more preferably, at least about 10:1. The media comprises a material selected from the group consisting of zirconium hydroxide, titanium hydroxide, hafnium hydroxide and combinations thereof. The solution is exposed to the media effective to remove therefrom species comprising an As+3 component in nonionic form. The solution with the species removed is passed from the media.
A preferred embodiment of the present invention is directed to a method of treating water, comprising passing water into contact with the layer of media having an aspect ratio of at least 1:1, more preferably, at least about 10:1, wherein the media comprises zirconium hydroxide. The water is exposed to the media effective to remove therefrom species comprising an As+3 component in nonionic form. The water with the species removed is passed from the media.
Another embodiment of the present invention is directed to a device for treating an aqueous solution. The device comprises a housing and at least one porous layer is constructed and arranged to partition the housing into an untreated solution region and a treated solution region. A layer of the inventive media having an aspect ratio of at least 1:1, more preferably, at least about 10:1, is disposed in the untreated region adjacent the at least one porous layer. A seal engages the porous layer to prevent fluid communication between the untreated region and the treated region other than through the at least one porous layer. An inlet is adapted to communicate a source of an untreated aqueous solution to the untreated region of the housing. An outlet discharges a treated aqueous solution from the treated region of the housing. In one form of the invention, the layer is wetted at all times during solution processing.
More specific device features are that the inlet is adapted to be fastened to a spout leading from a tube well or water line of a sink. The layer of media and the at least one porous layer may take a variety of shapes, including generally cylindrical or planar. The device may comprise a cartridge that is adapted to be removably fastened in the housing, the cartridge comprising the layer of media and the at least one porous layer. The porous layer may include a plurality of pleats, or it may be flat.
Yet another embodiment of the invention is directed to a filter press type device for treating an aqueous solution, comprising a plurality of sets of press members. Each set of press members comprises a hollow frame member, a layer of the inventive media, a filter cloth and a back plate. The hollow frame member includes a central opening for passing fluid therethrough. A fluid inlet leads to the frame member so as to communicate an untreated aqueous solution to the central opening. The media is disposed so as to obturate the central opening. A back plate has indentations formed therein, the back plate being configured and arranged so as to obturate the central opening. A porous layer is disposed between the back plate and the frame member so as to obturate the central opening. A fluid outlet leads from the back plate so as to discharge a treated aqueous solution from the back plate. Support and movement of the press members is facilitated in a known manner. The press members are pressed together under pressure so as to form a water tight seal between the press members of each set and between adjacent sets of press members. When the press members are compressed, an untreated aqueous solution enters the frame member and passes through the media where toxic or undesirable species are removed. The solution with removed species passes through the filter cloth which prevents movement of the media. The treated solution travels via the indentations to the outlet tube where it is discharged from the press.
Much has been said up to this point about the ability of the inventive media to perform in a manner unlike ion exchange materials in its affinity for nonionic arsenite. However, the inventive media removes ionic species from water as well. In regards to the removal of ionic species in accordance with the present invention, a method of treating an aqueous solution comprises passing the solution into contact with a layer of the inventive media having an aspect ratio of at least 1:1, more preferably, at least about 10:1. The aqueous solution is exposed to the media effective to remove ionic species therefrom, and the aqueous solution with the species removed is passed from the media. The species that may be removed are selected from the group consisting of arsenate, selenate, chromate, borate, perchlorate, fluoride and combinations thereof.
Regarding particular features of the invention, the media is untreated with acid. The As+3 component is removed from the aqueous solution after a single exposure to the media so as not to be present in an amount greater than about 10 parts per billion, even more preferably, in an amount not greater than about 1 part per billion. The As+3 component is removed directly from the aqueous solution without conversion to a species comprising As+5 prior to removal of the As+3 component. The aqueous solution is preferably groundwater treated so as to be suitable for drinking. The aqueous solution preferably has a pH ranging from about 6.5 to about 7.5. The water may be delivered from a tube well or from a source of water leading to a sink.
The media is in a form of a powder having a water content characterized by a loss on ignition of at least 40% when heated at 1000xc2x0 C. for 1 hour. The media may comprise zirconium hydroxide or titanium hydroxide. The zirconium hydroxide media has a density upon tamping to constant volume, of at least 0.7 g/ml. The species that may be removed are selected from the group consisting of arsenate, selenate, chromate, borate, perchlorate, fluoride and combinations thereof. A ratio of species removal, As+3 component/As+5 component, is at least 4:1. The As+3 component is removed to not greater than 10 ppb despite a presence of at least one competing species selected from the group consisting of sulphate, phosphate, nitrate, bicarbonate, iron, carbonate, nitrite, silicate, sulphite, chloride, bromide and iodide.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description and the accompanying drawings which illustrate, by way of example, the principles of the present invention.